This invention relates to a robotic paint application system for use in a potentially explosive atmosphere, such as a paint booth, and a method of protecting a paint robot having an electric motor in such atmosphere.
A conventional paint application system for mass production applications typically includes a plurality of rotary atomizers which are mounted on various fixtures to apply paint or other coatings to a substrate. The substrate, such an automotive body, is typically mounted on a conveyor which traverses the paint booth and is sprayed by the paint applicators. In one typical application, a plurality of overhead and side mounted rotary atomizers are mounted on a U-shaped frame assembly which moves on tracks with the vehicle body. The paint applicators, which may be conventional rotary atomizers or other conventional spray devices, may be mounted on robot arms to apply paint to all areas of the vehicle body as the vehicle body traverses the paint booth on a conveyor. The paint booth is generally enclosed because of the overspray and the potentially explosive atmosphere which may be created by the paint overspray. One example of a potentially explosive atmosphere is volatile organic compounds or vocs, including volatile organic solvents utilized as a solvent for paint. The paint overspray and solvents are continuously removed from the atmosphere of the paint spray booth by various recovery systems.
More recently, true robotic paint application systems are being used in mass production applications. A typical robotic paint applicator includes a base member, which may be mounted on the floor of the paint spray booth or mounted on a rail to traverse with the substrate mounted on a conveyor, an intermediate section or housing, typically pivotally or rotatably mounted on the base member, and a generally horizontal robot arm pivotally mounted on the intermediate member having a paint applicator, generally a rotary paint atomizer, at its distal end. The intermediate section and the robot arm are then manipulated by motors generally connected to a computer, to continuously move the paint applicator to apply paint over the substrate as the substrate is moved through the paint booth. The movement of the intermediate section and the robot arm may be controlled by hydraulic motors. However, hydraulic controls are expensive, complicated and subject to failure. Electric motors, particularly servomotors, have replaced hydraulic controls because servomotors provide better control at less cost and servomotors have less service problems. However, electric servomotors have a potential for sparking and thus create potential safety issues in a potentially explosive atmosphere, such as a paint booth applying liquid paint having an organic solvent. Conventional sealed explosion proof servomotors are not practical in this application because of the bulk and weight of such explosion proof motors.
The prior art has proposed flooding the section compartments or enclosures containing xe2x80x9cnon-explosion proof motorsxe2x80x9d with an xe2x80x9cinert gas,xe2x80x9d such as air or nitrogen, to prevent entry of the potentially combustible atmosphere in the paint booth, such as disclosed, for example, in U.S. Pat. No. 4,984,745. However, this approach has several problems. First, there are typically compartments within the enclosures, particularly including the housing of the servomotor. That is, this patent proposes to use conventional or xe2x80x9cnon-explosion proofxe2x80x9d servomotors having a housing which is not sealed or explosion proof. As will be understood by those skilled in this art, a conventional paint application system does not run continuously. That is, the paint application system is periodically shut down for shift changes, maintenance, etc., and the paint application line may be shut down for one or more eight hour shifts. Thus, potentially combustible gas from the paint booth will enter the robot housing enclosures and the compartments within the housing enclosures, including the motor housings, when the supply of non-combustible gas supplied to the base member is turned off, such as when the paint application system is idle. When the combustible gas enters the housing enclosures containing the non-explosion proof servomotors, the combustible gas may also enter the housings of the servomotors creating a potential explosion hazard. However, flooding the housing enclosures containing the servomotors with a non-combustible gas will not necessarily purge combustible gas in the servomotor housings, creating a potentially explosive atmosphere in the motor housings. Further, circulating the non-combustible gas to the base or lower housing enclosures to the other enclosures of the robot may not thoroughly purge the components within the enclosures. Thus, there is a need for an improved robotic paint applicator and method of protecting a paint robot having electric motors from explosion in an enclosed paint booth having a potentially combustible atmosphere. The robotic paint applicator system and method of this invention solves this problem in a simple, cost effective manner.
The robotic paint applicator and method of this invention begins with the electric motor which, as set forth above, is preferably an electric servomotor to provide accurate and fast control of the robotic paint applicator. The electric motor includes the conventional components of an electric motor, including a stator, rotor and drive shaft. The explosion proof electric motor utilized in the paint applicator of this invention includes a relatively air-tight housing surrounding the electrical components of the motor, wherein the housing includes a gas inlet and a gas outlet spaced from the gas inlet. A source of non-combustible gas under pressure is connected to a gas inlet of the motor housing and the non-combustible gas thus creates a positive pressure of non-combustible gas within the motor housing, purging the motor housing and preventing entry of potentially combustible gas into the motor housing. Thus, the servomotors utilized in the robotic paint applicator of this invention are explosion proof. Further, the enclosures of the sections of the robotic paint applicator containing the explosion proof motors are generally or nearly air tight, such that the non-combustible gas is received from the gas outlet of the motor housings into the robot housing enclosures, providing those housing enclosures with non-combustible gas, thereby creating explosion proof robot housing enclosures.
As set forth below in regard to the method of this invention, the non-combustible gas, such as air, is initially supplied to the motor housings with sufficient pressure, such as 4 bar, to purge not only the motor housing, but also the robot section enclosure containing the electric motor and the further electrical parts or components contained within the enclosure. Following purging, the non-combustible gas is supplied to the motor housing at a lesser pressure, preferably at least 0.8 mbar, to maintain a positive pressure of non-combustible gas greater than atmospheric in the motor housings and the robot section enclosures. In a preferred embodiment, the motor housing includes an inlet which receives the non-combustible gas and a tube which communicates with the electrical components of the electric motor including the windings and rotor, and an outlet or exit port preferably having a diameter greater than the inlet. In the disclosed embodiment, the gas outlet is xe2x80x9ca flame restrictorxe2x80x9d filter. As used herein, the term xe2x80x9cexplosion proof electric motorxe2x80x9d means a conventional electric motor, particularly including an electric servomotor, including an enclosed housing having gas inlets and outlets as described above, but excludes xe2x80x9cnon-explosion proofxe2x80x9d electric motors.
The robotic paint applicator of this invention includes a housing enclosure, preferably a substantially or nearly air tight robot housing enclosure, containing an explosion proof electric motor and a robot arm mounted on the robot enclosure having a paint applicator on a distal end of the robot arm. As set forth above, the robot arm of the paint applicator generally also includes a wrist or wrist mechanism and the applicator is typically a rotary paint atomizer, but may be any type of applicator. The robot paint applicator is typically located in an enclosed paint booth having a potentially combustible atmosphere including, for example, a solvent containing vocs. As described above, the explosion proof electric motor includes a motor housing having a gas inlet, a gas outlet and a source of non-combustible gas under pressure, preferably located outside the paint spray booth, connected to the gas inlet of the motor housing, pressurizing the motor housing with non-combustible gas for purging and preventing the potentially combustible atmosphere from entering the motor housing. The gas outlet of the motor housing directs non-combustible gas into the nearly air-tight robot enclosure containing the electric motor, creating a positive pressure of non-combustible gas within the robot enclosure and preventing the potentially combustible gas from entering the robot enclosure, thereby protecting other electrical components within the robot enclosures, such as wires, switches and the like, from being exposed to the potentially combustible atmosphere of the paint spray booth. In a typical application of the robotic paint applicator of this invention, each of the robot enclosures or section components of the base, the intermediate sections or members and the robot arm includes at least one electric motor for manipulating the paint applicator and at least one of the robot enclosures may include a plurality of explosion proof electric motors. In such embodiments, each of the explosion proof electric motors include a motor housing having a gas inlet and a gas outlet and the robotic paint applicator includes a plurality of conduits, each of which is connected to the source of non-combustible gas under pressure, such that each of the motor housings is directly flushed or purged with clean non-combustible gas directly from the source and each of the robot enclosures or compartments is maintained at a positive pressure of non-combustible gas received through the gas outlet of the motor housings. In one preferred embodiment, the robot enclosures containing the explosion proof electric motors are in fluid communication, having conduits between adjacent enclosures, such that non-combustible gas received from the explosion proof motors is directed from one robot enclosure to the next robot enclosure to an outlet in the lower enclosure or base member, assuring complete purging and maintenance of a positive pressure of non-combustible gas within each of the enclosures.
The preferred method of protecting a paint robot having an electric motor from explosion in an enclosed paint spray booth having a combustible atmosphere of this invention includes first enclosing an explosion proof electric motor or electric motors and controls in a substantially air-tight enclosure. As used herein, the term xe2x80x9csubstantially air-tightxe2x80x9d means that the enclosure or compartment is substantially completely enclosed as is conventional for such enclosures, such that a positive gas pressure may be maintained in the robot housing enclosure. The method of this invention further includes providing an explosion proof electrical motor with a substantially air-tight motor housing having a gas inlet and a gas outlet preferably spaced from the gas inlet as described above.
The method of this invention then includes purging the motor housing and the robot enclosure containing the explosion proof electric motor by supplying a non-combustible gas, preferably air, under pressure to the gas inlet of the motor housing under sufficient pressure to circulate the non-combustible gas through the motor housing and through the gas outlet into the robot section enclosure, purging the motor housing and the robot section enclosure of potentially combustible gas. In a preferred embodiment, during the purging step, air is supplied to the motor housing at a pressure of about 4 bars and the volume of air supplied to the motor housing is preferably at least 5 times or between about 5 and 10 times the volume of the motor housing and the robot housing enclosure which contains the explosion proof electric motor. This volume and pressure assures complete purging of combustible gas from the motor housing and the robot housing enclosure. The explosion proof electric motors can then be safely operated to control the paint robot. Finally, the method of this invention includes continuing to supply the non-combustible gas to the inlet of each of the motor housings at a lesser pressure sufficient to maintain the motor housings and the robot housing enclosures at a positive pressure, thereby preventing combustible gas from entering the housing enclosures and the motor housings. A pressure of about 0.8 mbar is generally sufficient to maintain a positive pressure of non-combustible gas in the motor housings and the robot enclosures containing the motor housings. As set forth above, where the paint robot includes a plurality of explosion proof electric motors, the method of this invention includes separately purging each of the motor housings and the robot housing enclosures by delivering non-combustible gas under pressure from a source of non-combustible gas, preferably located outside the paint booth, separately connected to each of the inlets of the motor housings as described above.
The robot paint applicator and method of this invention will be more fully understood from the following description of the preferred embodiments, the appended claims and the drawings, a brief description of which follows.